|
![]() SelfdischargeSelfdischarge and leakage currentThe selfdischarge is an important parameter for the applications in which the supercapacitors are not connected to an electrical network which could maintain its state of charge. In those applications the device is supposed to be able to deliver power with a performance not deteriorated by the rest time. A typical example is the cranking of a car engine after a week spent at rest in an airport parking lot. In this case, it's necessary that the storage device maintains its tension as high as possible because the available power and the stored energy decline with the square of this tension.
The voltage drop with the time of a charged capacitor in a floating mode may be due to different discharging mechanisms, which are: - the leakage current, - the charge redistribution The charge redistribution corresponds to the displacement of charges from an easy access charged area of the electrode with a low access time constant, to a more difficult one, with a higher time constant. The charge redistribution, contrarily to the selfdischarge, may lead to a voltage increase measured at the capacitor terminal. After a fast capacitor discharge, the voltage may increase a little bit due to the arrival of charges of slow areas. The leakage current may be due to redox reactions, ionic charge diffusion or/and electronic partial discharge through the separator. The selfdischarge amplitude is determined, either by measuring the current necessary to feed to keep a constant voltage (in the microamperes range), or by recording the capacitor voltage as a function of the time. The diagnostic can be performed according to the voltagetime dependency. If the voltage drops with an unwanted logarithmic law, U(t) vs. log t (see fig. 1), the mechanisms are controlled by Faradic self-discharge from overcharge or from impurities redox reactions. If the voltage drops with a square root function of the time, U(t) vs. t (see fig. 2), the mechanisms are controlled by diffusion.
Recent measurements have shown that 10 years are necessary to fully discharge a supercapacitor of the new generation at room temperature. The supercapacitor selfdischarge performance is the result of a compromise with its power capability. The manufacturers could use a thicker separator to improve the voltage retention but this operation would increase in the same time the series ionic resistance.
Source: Garmanage: Roland Gallay [1]R. Kötz, et al., Journal of Power Sources 174 (2007) 264271, "Voltage balancing: Long term experience with the 250V supercapacitor module of the hybrid fuel cell vehicle HY-LIGHT (Michelin +PSI)
|